Firefox is an MSAA application and currently does not implement UI Automation (UIA). A project was started to add UIA support to Firefox, see this blog post for details. Also you can track the progress of UIA implementation on bugzilla
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Paulius UzaNov 5 '12 at 13:56

I've checked out the link you specified and can get Narrator to read everything except the "meat" of the application. So it can navigate menus and toolbars, but can't even get it read the text of a MS Word 2010 Document. Have you actually had success getting Narrator to read anything?
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Joel BFeb 5 '13 at 17:09